1. Field Of The Invention
The present invention relates to an airlock used to feed a material continuously between zones of different gas pressure while obstructing the free flow of gas. It particularly relates to such an airlock for use in delivering tobacco or other friable material.
2. Description Of The Related Art
Many manufacturing operations use differential gas pressure, generated either by suction or forced gas, to move material from one location to another. For example, in the production of tobacco products, flowing air is often used to transport and/or classify particles of tobacco. At the terminus of such a transport or classification process, it is often desired to separate the tobacco particles from the airflow. Such separation is generally accomplished today by a rotary airlock. Such an airlock typically may comprise a funnel-shaped feed inlet, a housing having therein a chamber in the shape of a horizontal cylinder with a transport wall and a return wall, a vaned rotor which coaxially rotates within the cylindrical chamber, and an outlet at the bottom of the chamber. The vaned rotor may have flexible, selvedge-tipped vane edges or hard-tipped vane edges. Examples of similar airlocks are referred to in U.S. Pat. Nos. 2,643,768; 2,667,174; 2,701,570; 2,988,213; 3,043,315; 3,046,998; 3,116,238; 3,360,125; 3,362,414; 3,409,025; 3,513,858; 3,608,716; 3,655,043; 3,665,932; 4,166,027; 4,259,032; 4,264,238; 4,308,876; and 4,446,876.
There are problems with such rotary airlocks, primarily an undesired degree of breakage of tobacco strands, and secondarily a degree of abrasive wear to the airlock itself. As each vane sweeps up from the return wall of the cylinder into the tobacco entering the rotary airlock, the vane begins a shearing action between the falling tobacco strands. This shearing action may break tobacco strands.
As the vane edge passes through the incoming tobacco, tobacco strands may become momentarily impinged on the vane edge. This may result in breakage of tobacco strands due to the fast motion of the vane edge relative to the tobacco strands. When the vane edge then rotates toward the transport wall of the airlock chamber, it creates a high-shear point which may cause further strand breakage or even possibly cause the rotor to jam. As the vane edge sweeps along the transport wall, tobacco between adjacent vanes is thrown outward toward the wall by centrifugal force. Because the vanes move from a horizontal direction to a vertical direction, gravity also causes the tobacco to tumble in the pocket between adjacent vanes toward the transport wall.
As the vane edge continues its motion, there is a grinding action between the vane edge and the transport wall which may further damage the tobacco and additionally wear both the transport wall and the vane edge, particularly when foreign matter such as sand is present. At the outlet, interaction between driven and falling tobacco may shear and break more tobacco.
Another type of valve for maintaining an airlock while transferring material by differential air pressure is an inflatable valve. An example of such an airlock is referred to in U.S. Pat. No. 4,286,910 to Conrad. An inflatable valve uses multiple inflatable elements which cooperate to transfer material between zones of the same or different pressure by peristaltic action. The main disadvantage of such a valve is its relative complexity, which limits its practical application. This is due to limited product flow rates achievable through the valve, a substantial risk of mechanical problems associated with the operation of the valve, potential loss of sealing between zones from product buildup on sealing surfaces, and product degradation from compression during sealing.
Single endless belts also have been used in airlocks. For example, U.S. Pat. No. 540,155 to Dula et al. refers to an endless belt provided with cross pieces which conducts tobacco leaves from a box and discharges them into a pneumatic conveyor. The discharge of air past the endless belt is said to be prevented on one end by a close fit between the cross pieces and a rounded end of the box and on the other end by flexible strips lapping on the cross pieces.
If used for separating tobacco, the contact between the flexible strips and the cross pieces would cause shearing of tobacco caught on the edges of the cross pieces. The contact would also cause wear of both the flexible strips and the cross pieces at the point of contact. This single belt system involves relative motion between the stationary flexible strips and the moving leaves as the leaves fall by gravity. This relative motion results in further breakage of the conveyed material.
In the Dula et al. device, sealing is intended to occur in the curved portions of the airlock. The seal is horizontal to vertical (tobacco discharge side) and vertical to horizontal (return side) similar to a rotary valve. Because of this arrangement, gravity and centrifugal force would cause the tobacco leaves to contact the flexible sealing strips. This contact would damage the tobacco.
Finally, because the seal between the flexible strips and the cross pieces is not continuous, substantial air leakage would result. The air leakage would produce further breakage of the conveyed material by moving air in the opposite direction of the discharging leaves (suction system) or by creating pockets of turbulence (positive pressure system). The airlock would rapidly jam under these conditions.
U.S. Pat. No. 3,091,244 to Molins et al. and U.S. Pat. No. 3,514,159 to Labbe refer to an endless band forming one wall of a channel. Tobacco moving through the channel forms an airlock. There is relative motion between the non-moving walls of the channel and the tobacco confined thereby. The airseals in Molins et al. and Labbe are accomplished along a portion of the belt which is not horizontal.